The present invention relates to a method for preparing a moisture absorbent material and the resulting moisture absorbent product. In particular, it relates to radiation induced graft polymerization of substrates to provide highly swellable materials and conversion of such materials to swollen, additive-dispensing products.
In the past it has proven difficult to substantially enhance the moisture absorbing ability of substrates, while maintaining their structural integrity. Highly absorbent cellulosic substrates, for example, are required to meet the ever-increasing need for diapers, tampons, sanitary napkins, medical sponges, bandages, cloth wipes, and the like. Such substrates may be required to absorb over 100 times their weight of aqueous fluid. Further, in the consumer and medical fields the need for storage-stable, water-swellable matrices adapted to carry aqueous solutions of easily volatized additives is greatly expanding.
It has long been known that synthetic polymers can be made more hydrophilic by radiation induced graft polymerization. Fabrics and films formed from addition polymers have been disclosed in U.S. Pat. No. 3,188,165, as possible substrates for the electron beam grafting thereto of unsaturated carboxylic acids or their preformed organic salts, in a single operation. Cellulosic substrates are not disclosed. Cross-linking momomers were not utilized. The typical add-on (weight gain of grafted polymer for the substrate) was only from about 25 to 30 weight percent. For practical purposes this is insufficient to provide enhanced moisture absorption and wet strength required for materials which are to be used as diapers, tampons and the like.
It is believed that the low dose rates imparted by the early prior art radiation sources, such as the Van de Graff electron accelerator, conventional X-ray equipment and radioactive isotopes, as cobalt 60, simply were insufficient to initiate the requisite degree of free radical generation to form the engrafted polymer network required. For commercial and other purposes, it has also been desired to provide a grafting process by which a sample can be properly irradiated to cause formation of sufficient active sites on the substrate and polymerization of monomer at production line speeds, such that the irradiated material can be quickly and efficiently worked into the desired end product.
Graft polymerization of natural polymeric material, including cellulosic fibers, with acrylic acid monomer is disclosed in U.S. Pat. No. 3,514,385. Low add-on, on the order of 25%, is reported. Irradiation is at low dosage rates. No cross-linking monomers are employed. Acrylic acid grafted substrates are converted to the sodium salt form in a separate step; no direct grafting of acrylate salts is disclosed.
In U.S. Pat. No. 3,799,881 polyester fiber is impregnated with a solution of from 5 to 20% acrylate salt monomer and 80% to 95% cross-linking monomer and irradiated to graft the resulting polymer to the polyester substrate. The weight increase of grafted polymer is very low, on the order of only 2-10%. The resulting polyester product is merely a "coated" polyester fiber material. The results achieved in this patent for sodium methacrylate were comparable to those achieved for sodium acrylate. This is in direct contrast to the present invention, wherein it has been found that high add-on of methacrylate (on the order of 60% or more), is not possible by conventional radiation grafting. Also, at the high ratio of cross-linking agent to monomer employed in U.S. Pat. No. 3,799,881, the resulting graft polymer is not swellable and exhibits a very low water absorption compared to that desired by industry.
In U.S. Pat. No. 3,247,133 polyethylene film is irradiated by an electron beam in the presence of monomer to form an ion exchange membrane. Add-ons of monomer are less than half the amount required for the proper degree of absorbability needed for diapers, tampons and the like. Cross-linking agents are mentioned as optional ingredients and in amounts (20%) which would make the material too rigid for normal use. No acrylate salts are disclosed.
Cross-linked hydrophilic finishes for polyester, utilizing ethoxylated acrylate monomer irradiated with an electron-beam, is disclosed in a reprinted article, originally appearing on pages 37-43 of Vol. 9, No. 1, Jan. 1977, in Textile Chemist and Colorist, published by the American Association of Textile Chemists and Colorists. The add-on of cross-linked monomer was very low, less than only 17%, with retained water being on the order of only 4 times the weight of fabric. For practical purposes, the material should retain on the order of 50-100 times its weight of water.
A proposal to increase water absorption in cellulosic materials employing electron beam induced graft polymerization of acrylic acid is disclosed in U.S. Pat. No. 4,036,588. A post-decrystallization step of 2 hours duration was essential to achieve a high degree of water absorption. No acrylate salts were employed. No cross-linking agents were disclosed. The radiation dose rate was too low for practical purposes.
Radiation induced grafts of up to about 22% acrylic acid on nylon, have been thereafter converted to the sodium acrylate salt by after-treatment with sodium carbonate, Magat et al., J. Polym. Sc., Part C, No. 4, pp. 615-629 (1963). No cross-linking agents were employed.
In the past it has also been reported that radiation was a disappointing tool for cross-linking types of processing, particularly in the case of cellulosics and other fibers which degrade rather than cross-link on irradiation, Am. Dyestuff Reporter, Dec. 2, 1968, pp. 91-100, (93). It was recognized by the art that graft polymerization, initiated by ionizing radiation, was unpredictable and that in each case the amount of radiation from a given source had to be empirically determined for the particular result sought, col. 3, lines 72-75, U.S. Pat. No. 3,252,880.